In order to provide an optimal sensitivity, RFID transponders and RFID tags should have their antenna resonant circuit well tuned to the reader's resonance frequency, in order to achive a maximum power transfer as soon as the transponder or tag enters an area of coverage of the RFID reader. Since passive RFID tags do not have an internal power supply, the electrical power required to activate the tag's integrated circuit has to be entirely drawn from the reader's RF-field.
In order to receive sufficient electrical power, the resonance frequency of an RFID tag's antenna resonant circuit should precisely match the frequency of the reader.
However, manufacturing variability of various low cost resonant circuit components results in a substantial variation of a default resonance frequency from tag to tag.
Also, depending on the specific area of application, the resonance frequency of the RFID tag's antenna resonant circuit may vary, e.g. due to environmental influences. For instance, if RFID tags are used for a product labelling or identification, wherein the RIFD tag is attached to the product or to the product package, the product and product packaging itself will have a non-neglectable influence on the default resonance frequency of the tag's antenna resonant circuit.
Since passive-mode RFID tags must draw their power from the applied RF-field in order to reach a maximum communication distance, these tags should be extremely sensitive and should activate and “power-up” with as low an applied RF-field as possible. An integrated circuit, hence an operating unit being adapted to interact with the RF-field therefore must be “centered” in frequency selection and in its optimum internal bias and operating conditions. However, due to normal manufacturing tolerances or due to a specific tag environment, the antenna resonant circuit of the tag comprises a default resonance frequency, that does not sufficiently match the reader's frequency.
Once the RFID tag has received enough power to activate itself, it is also able to retrieve trim values from a non-volatile memory and to perform a required trim and tuning procedure in order to match its resonance frequency to the reader's frequency. However, in order to perform such a trim or tuning procedure, the tag's operating unit, typically embodied as an integrated circuit, has to activate, which at a given communication distance is not possible as long as the tag's antenna resonant circuit's resonance frequency is offset from the reader's frequency.
Existing solutions either require on-board non-volatile memory in order to install trimmed values for resonance frequency tuning. Alternatively, various frequency scanning systems are suggested, which are adapted to scan some frequency range in order to find the frequency of the RFID reader. However, either method requires that the transponder or tag receives adequate power to access the non-volatile memory and/or to perform logic operations, which in turn are only executable, once the operating unit of the RFID tag has activated or powered-up. Consequently, existing tag architecture requires the resonant circuit to be as close to the reader's frequency prior to application of power.
Document U.S. Pat. No. 7,132,946 B1 discloses radio frequency identification tags that dynamically vary the resonance frequency to reduce or to eliminate the potential effects of electromagnetic “tag-to-tag” coupling. This RFID tag includes a switch being adapted to switch between various inductive elements in order to selectively include inactive elements as additional loops for an antenna. This switch may comprise a low power microelectromechanical system (MEMS) switch, a capacity switch or other switching components. This switch may either be designed to automatically switch between inductive elements, e.g. during a single power up-cycle.
When the RFID tag receives enough power to activate the MEMS switch, the MEMS switch changes position in an attempt to draw enough current/voltage from the RF energy to power an integrated circuit. The MEMS switch may change position after each time RFID tag is powered down. In this manner, the RFID tag would alternate resonating at two different frequencies at every other power-up cycle.